Network Working Group N. Haller
Request for Comments: 1938 Bellcore
Category: Standards Track C. Metz
Kaman Sciences Corporation
May 1996
A One-Time Password System
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1.0 ABSTRACT
This document describes a one-time password authentication system
(OTP). The system provides authentication for system access (login)
and other applications requiring authentication that is secure
against passive attacks based on replaying captured reusable
passwords. OTP evolved from the S/KEY (S/KEY is a trademark of
Bellcore) One-Time Password System that was released by Bellcore and
is described in references [3] and [5].
2.0 OVERVIEW
One form of attack on networked computing systems is eavesdropping on
network connections to obtain authentication information such as the
login IDs and passwords of legitimate users. Once this information is
captured, it can be used at a later time to gain access to the
system. One-time password systems are designed to counter this type
of attack, called a "replay attack" [4].
The authentication system described in this document uses a secret
pass-phrase to generate a sequence of one-time (single use)
passwords. With this system, the user's secret pass-phrase never
needs to cross the network at any time such as during authentication
or during pass-phrase changes. Thus, it is not vulnerable to replay
attacks. Added security is provided by the property that no secret
information need be stored on any system, including the server being
protected.
The OTP system protects against external passive attacks against the
authentication subsystem. It does not prevent a network eavesdropper
from gaining access to private information and does not provide
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RFC 1938 A One-Time Password System May 1996
protection against either "social engineering" or active attacks [9].
3.0 INTRODUCTION
There are two entities in the operation of the OTP one-time password
system. The generator must produce the appropriate one-time password
from the user's secret pass-phrase and from information provided in
the challenge from the server. The server must send a challenge that
includes the appropriate generation parameters to the generator, must
verify the one-time password received, must store the last valid
one-time password it received, and must store the corresponding one-
time password sequence number. The server must also facilitate the
changing of the user's secret pass-phrase in a secure manner.
The OTP system generator passes the user's secret pass-phrase, along
with a seed received from the server as part of the challenge,
through multiple iterations of a secure hash function to produce a
one-time password. After each successful authentication, the number
of secure hash function iterations is reduced by one. Thus, a unique
sequence of passwords is generated. The server verifies the one-time
password received from the generator by computing the secure hash
function once and comparing the result with the previously accepted
one-time password. This technique was first suggested by Leslie
Lamport [1].
4.0 REQUIREMENTS TERMINOLOGY
In this document, the words that are used to define the significance
of each particular requirement are usually capitalized. These words
are:
- MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of the specification.
- SHOULD
This word or the adjective "RECOMMENDED" means that there might
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the
case carefully weighed before taking a different course.
- MAY
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor might choose to include the item
because a particular marketplace requires it or because it
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RFC 1938 A One-Time Password System May 1996
enhances the product, for example; another vendor may omit the
same item.
5.0 SECURE HASH FUNCTION
The security of the OTP system is based on the non-invertability of a
secure hash function. Such a function must be tractable to compute in
the forward direction, but computationally infeasible to invert.
The interfaces are currently defined for three such hash algorithms,
MD4 [2] and MD5 [6] by Ronald Rivest, and SHA [7] by NIST. All
conforming implementations of both server and generators MUST support
MD5. They SHOULD support SHA and MAY also support MD4. Clearly, the
generator and server must use the same algorithm in order to
interoperate. Other hash algorithms may be specified for use with
this system by publishing the appropriate interfaces.
The secure hash algorithms listed above have the property that they
accept an input that is arbitrarily long and produce a fixed size
output. The OTP system folds this output to 64 bits using the
algorithms in the Appendix A. 64 bits is also the length of the one-
time passwords. This is believed to be long enough to be secure and
short enough to be entered manually (see below, Form of Output) when
necessary.
6.0 GENERATION OF ONE-TIME PASSWORDS
This section describes the generation of the one-time passwords.
This process consists of an initial step in which all inputs are
combined, a computation step where the secure hash function is
applied a specified number of times, and an output function where the
64 bit one-time password is converted to a human readable form.
Initial Step
In principle, the user's secret pass-phrase may be of any length.
To reduce the risk from techniques such as exhaustive search or
dictionary attacks, character string pass-phrases MUST contain at
least 10 characters (see Form of Inputs below). All
implementations MUST support a pass-phrases of at least 63
characters. The secret pass-phrase is frequently, but is not
required to be, textual information provided by a user.
In this step, the pass phrase is concatenated with a seed that is
transmitted from the server in clear text. This non-secret seed
allows clients to use the same secret pass-phrase on multiple
machines (using different seeds) and to safely recycle their
secret pass-phrases by changing the seed.
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RFC 1938 A One-Time Password System May 1996
The result of the concatenation is passed through the secure hash
function and then is reduced to 64 bits using one of the function
dependent algorithms shown in Appendix A.
Computation Step
A sequence of one-time passwords is produced by applying the
secure hash function multiple times to the output of the initial
step (called S). That is, the first one-time password to be used
is produced by passing S through the secure hash function a number
of times (N) specified by the user. The next one-time password to
be used is generated by passing S though the secure hash function
N-1 times. An eavesdropper who has monitored the transmission of a
one- time password would not be able to generate the next required
password because doing so would mean inverting the hash function.
Form of Inputs
The secret pass-phrase is seen only by the OTP generator. To allow
interchangeability of generators, all generators MUST support a
secret pass-phrase of 10 to 63 characters. Implementations MAY
support a longer pass-phrase, but such implementations risk the
loss of interchangeability with implementations supporting only
the minimum.
The seed MUST consist of purely alphanumeric characters and MUST
be of one to 16 characters in length. The seed is a string of
characters that MUST not contain any blanks and SHOULD consist of
strictly alphanumeric characters from the ISO-646 Invariant Code
Set. The seed MUST be case insensitive and MUST be internally
converted to lower case before it is processed.
The sequence number and seed together constitute a larger unit of
data called the challenge. The challenge gives the generator the
parameters it needs to calculate the correct one-time password
from the secret pass-phrase. The challenge MUST be in a standard
syntax so that automated generators can recognize the challenge in
context and extract these parameters. The syntax of the challenge
is:
otp-<algorithm identifier> <sequence integer> <seed>
The three tokens MUST be separated by a white space (defined as
any number of spaces and/or tabs) and the entire challenge string
MUST be terminated with either a space or a new line. The string
"otp-" MUST be in lower case. The algorithm identifier is case
sensitive (the existing identifiers are all lower case), and the
seed is case insensitive and converted before use to lower case.
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RFC 1938 A One-Time Password System May 1996
If additional algorithms are defined, appropriate identifiers
(short, but not limited to three or four characters) must be
defined. The currently defined algorithm identifiers are:
md4 MD4 Message Digest
md5 MD5 Message Digest
sha1 NIST Secure Hash Algorithm Revision 1
An example of an OTP challenge is: otp-md5 487 dog2
Form of Output
The one-time password generated by the above procedure is 64 bits
in length. Entering a 64 bit number is a difficult and error prone
process. Some generators insert this password into the input
stream and some others make it available for system "cut and
paste." Still other arrangements require the one-time password to
be entered manually. The OTP system is designed to facilitate this
manual entry without impeding automatic methods. The one-time
password therefore MAY be converted to, and all servers MUST be
capable of accepting it as, a sequence of six short (1 to 4
letter) easily typed words that only use characters from ISO-646
IVCS. Each word is chosen from a dictionary of 2048 words; at 11
bits per word, all one-time passwords may be encoded.
The two extra bits in this encoding are used to store a checksum.
The 64 bits of key are broken down into pairs of bits, then these
pairs are summed together. The two least significant bits of this
sum are encoded in the last two bits of the six word sequence with
the least significant bit of the sum as the last bit encoded. All
OTP generators MUST calculate this checksum and all OTP servers
MUST verify this checksum explicitly as part of the operation of
decoding this representation of the one-time password.
Generators that produce the six-word format MUST present the words
in upper case with single spaces used as separators. All servers
MUST accept six-word format without regard to case and white space
used as a separator. The two lines below represent the same one-
time password. The first is valid as output from a generator and
as input a server, the second is valid only as human input to a
server.
OUST COAT FOAL MUG BEAK TOTE
oust coat foal mug beak tote
Interoperability requires that all OTP servers and generators use
the same dictionary. The standard dictionary was originally
specified in the "S/KEY One Time Password System" that is
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RFC 1938 A One-Time Password System May 1996
described in RFC 1760 [5]. This dictionary is included in this
document as Appendix C.
To facilitate the implementation of smaller generators,
hexadecimal output is an acceptable alternative for the
presentation of the one-time password. All implementations of the
server software MUST accept case-insensitive hexadecimal as well
as six-word format. The hexadecimal digits may be separated by
white space so servers are REQUIRED to ignore all white space. If
the representation is partitioned by white space, leading zeros
must be retained. Examples of hexadecimal format are:
Representation Value
3503785b369cda8b 0x3503785b369cda8b
e5cc a1b8 7c13 096b 0xe5cca1b87c13096b
C7 48 90 F4 27 7B A1 CF 0xc74890f4277ba1cf
47 9 A68 28 4C 9D 0 1BC 0x479a68284c9d01bc
In addition to accepting six-word and hexadecimal encodings of the
64 bit one-time password, servers SHOULD accept the alternate
dictionary encoding described in Appendix B. The six words in
this encoding MUST not overlap the set of words in the standard
dictionary. To avoid ambiguity with the hexadecimal
representation, words in the alternate dictionary MUST not be
comprised solely of the letters A-F. Decoding words thus encoded
does not require any knowledge of the alternative dictionary used
so the acceptance of any alternate dictionary implies the
acceptance of all alternate dictionaries. Words in the
alternative dictionaries are case sensitive. Generators and
servers MUST preserve the case in the processing of these words.
In summary, all conforming servers MUST accept six-word input that
uses the Standard Dictionary (RFC 1760 and Appendix C), MUST
accept hexadecimal encoding, and SHOULD accept six-word input that
uses the Alternative Dictionary technique (Appendix B). As there
is a remote possibility that a hexadecimal encoding of a one-time
password will look like a valid six-word standard dictionary
encoding, all implementations MUST use the following scheme. If a
six-word encoded one-time password is valid, it is accepted.
Otherwise, if the one-time password can be interpreted as
hexadecimal, and with that decoding it is valid, then it is
accepted.
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RFC 1938 A One-Time Password System May 19967.0 VERIFICATION OF ONE-TIME PASSWORDS
An application on the server system that requires OTP authentication
is expected to issue an OTP challenge as described above. Given the
parameters from this challenge and the secret pass-phrase, the
generator can compute (or lookup) the one-time password that is
passed to the server to be verified.
The server system has a database containing, for each user, the one-
time password from the last successful authentication or the first
OTP of a newly initialized sequence. To authenticate the user, the
server decodes the one-time password received from the generator into
a 64-bit key and then runs this key through the secure hash function
once. If the result of this operation matches the stored previous
OTP, the authentication is successful and the accepted one-time
password is stored for future use.
8.0 PASS-PHRASE CHANGES
Because the number of hash function applications executed by the
generator decreases by one each time, at some point the user must
reinitialize the system or be unable to authenticate.
Although some installations may not permit users to initialize
remotely, implementations MUST provide a means to do so that does not
reveal the user's secret pass-phrase. One way is to provide a means
to reinitialize the sequence through explicit specification of the
first one-time password.
When the sequence of one-time passwords is reinitialized,
implementations MUST verify that the seed or the pass-phrase is
changed. Installations SHOULD discourage any operation that sends
the secret pass-phrase over a network in clear-text as such practice
defeats the concept of a one-time password.
Implementations MAY use the following technique for
[re]initialization:
o The user picks a new seed and hash count (default values may
be offered). The user provides these, along with the
corresponding generated one-time password, to the host system.
o The user MAY also provide the corresponding generated one
time password for count-1 as an error check.
o The user SHOULD provide the generated one-time password for
the old seed and old hash count to protect an idle terminal
or workstation (this implies that when the count is 1, the
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RFC 1938 A One-Time Password System May 1996
user can login but cannot then change the seed or count).
In the future a specific protocol may be defined for reinitialization
that will permit smooth and possibly automated interoperation of all
hosts and generators.
9.0 PROTECTION AGAINST RACE ATTACK
All conforming server implementations MUST protect against the race
condition described in this section. A defense against this attack
is outlined; implementations MAY use this approach or MAY select an
alternative defense.
It is possible for an attacker to listen to most of a one-time
password, guess the remainder, and then race the legitimate user to
complete the authentication. Multiple guesses against the last word
of the six-word format are likely to succeed.
One possible defense is to prevent a user from starting multiple
simultaneous authentication sessions. This means that once the
legitimate user has initiated authentication, an attacker would be
blocked until the first authentication process has completed. In
this approach, a timeout is necessary to thwart a denial of service
attack.
10.0 SECURITY CONSIDERATIONS
This entire document discusses an authentication system that improves
security by limiting the danger of eavesdropping/replay attacks that
have been used against simple password systems [4].
The use of the OTP system only provides protections against passive
eavesdropping/replay attacks. It does not provide for the privacy of
transmitted data, and it does not provide protection against active
attacks. Active attacks against TCP connections are known to be
present in the current Internet [9].
The success of the OTP system to protect host systems is dependent on
the non-invertability of the secure hash functions used. To our
knowledge, none of the hash algorithms have been broken, but it is
generally believed [6] that MD4 is not as strong as MD5. If a server
supports multiple hash algorithms, it is only as secure as the
weakest algorithm.
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RFC 1938 A One-Time Password System May 1996
GENERATOR ENCODING USING AN ALTERNATE DICTIONARY
The standard 6-word encoding uses the placement of a word in the
dictionary to represent an 11-bit number. The 64-bit one-time
password can then be represented by six words.
An alternative dictionary of 2048 words may be created such that
each word W and position of the word in the dictionary N obey the
relationship:
alg( W ) % 2048 == N
where
alg is the hash algorithm used (e.g. MD4, MD5, SHA1).
In addition, no words in the standard dictionary may be chosen.
The generator expands the 64-bit one-time password to 66 bits by
computing parity as with the standard 6-word encoding. The six 11-
bit numbers are then converted to words using the dictionary that
was created such that the above relationship holds.
SERVER DECODING OF ALTERNATE DICTIONARY ONE-TIME PASSWORDS
The server accepting alternative dictionary encoding converts each
word to an 11-bit number using the above encoding. These numbers are
then used in the same way as the decoded standard dictionary words
to form the 66-bit one-time password.
The server does not need to have access to the alternate dictionary
that was used to create the one-time password it is authenticating.
This is because the decoding from word to 11-bit number does not
make any use of the dictionary. As a result of the independence of
the dictionary, a server accepting one alternate dictionary accept
all alternate dictionaries.
Appendix C - Dictionary for Converting Between 6-Word and Binary
Formats
This dictionary is from the module put.c in the original Bellcore
reference distribution.
{ "A", "ABE", "ACE", "ACT", "AD", "ADA", "ADD",
"AGO", "AID", "AIM", "AIR", "ALL", "ALP", "AM", "AMY",
"AN", "ANA", "AND", "ANN", "ANT", "ANY", "APE", "APS",
"APT", "ARC", "ARE", "ARK", "ARM", "ART", "AS", "ASH",
"ASK", "AT", "ATE", "AUG", "AUK", "AVE", "AWE", "AWK",
"AWL", "AWN", "AX", "AYE", "BAD", "BAG", "BAH", "BAM",
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